Many modern day electronic devices comprise optical imaging devices (e.g., digital cameras) that use image sensors. Image sensors convert optical images to digital data that may represent the images. An image sensor may include an array of pixel sensors and supporting logic. The pixel sensors measure incident radiation (e.g., light), and the supporting logic facilitates readout of the measurements. One type of image sensor commonly used in optical imaging devices is a back-side illumination (BSI) image sensor. BSI image sensor fabrication can be integrated into conventional semiconductor processes for low cost, small size, and high through-put. Further, BSI image sensors have low operating voltage, low power consumption, high quantum efficiency, low read-out noise, and allow random access.